Embodiments of the invention relate generally to hybrid and electric vehicles and, more particularly, to a modular stacked direct current (DC) architecture traction system for hybrid and electric vehicles.
A hybrid electric vehicle (HEV) may combine an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Typically, the electric motor of an HEV is coupled between the internal combustion engine and the transmission to take advantage of the torque increase through the transmission. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in HEVs to be smaller and more fuel efficient.
A purely electric vehicle (EV) typically uses stored electrical energy to power an electric motor, which propels the vehicle. EVs may use one or more sources of stored electrical energy and are configured to use energy from an external source to re-charge the fraction battery or other storage devices. For example, a first source of stored energy (sometimes referred to as an “energy” source) may be used to provide longer-lasting energy while a second source of stored energy (sometimes referred to as a “power” source) may be used to provide higher-power for, for example, acceleration from standstill or boost during operation. First and second sources may include chemical-based batteries or may include ultracapacitors, as examples. Typically, the source(s) of electrical energy (energy and/or power batteries) in EVs are charged via a plug-in charger or other external energy source. With typically complete reliance on plug-in power, an EV may have increased energy storage capacity as compared to an HEV.
A plug-in hybrid vehicle (PHEVs) may include both an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery. Typically a PHEV is configured to use energy from an external source to re-charge the traction battery or other storage devices. Thus, with increased reliance on plug-in power, a PHEV may have increased energy storage capacity as compared to an HEV.
There are generally two types of PHEV: parallel and series. In a parallel PHEV arrangement, the electric motor is coupled between the internal combustion engine and the transmission, enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency, similar to an HEV. In a series PHEV arrangement, the electric motor is coupled between an energy storage device and the vehicle drive axle, while the internal combustion engine is coupled directly to the energy storage device and not to the vehicle drive axle. The series PHEV may also be referred to as an extended range electric vehicle (EREV), in reference to a purely electric drive system having energy augmentation to the energy storage system via the internal combustion engine and via, for instance, a liquid fuel storage system.
In general, EVs, HEVs, and PHEVs typically include regenerative braking to charge the charge storage devices during braking operations. Also, such vehicles may include on-road and off-road vehicles, golf cars, neighborhood electric vehicles, forklifts, and utility trucks as examples. These vehicles may use either off-board stationary battery chargers or on-board battery chargers to transfer electrical energy from a utility grid or renewable energy source to the vehicle's on-board traction battery.
While hybrid and electric vehicles offer many advantages, managing the stored energy efficiently and maintaining a good capacity of the energy storage (ES) elements over a defined period are important considerations when sizing these ES elements. A high voltage ES element having several cells in series often experiences less-than-optimal cell voltage balancing. Degradation of a single cell in a string of series cells affects the capacity of the entire string. Due to this reason, a low voltage ES with several parallel strings and low number of series strings is often preferred.
In contrast to the low voltage preferred on the ES, the EV motor typically meets the torque and efficiency requirements when operated at high voltages. Often, DC-DC boost converters are used to couple low voltage ES to the high voltage DC link from which the drive of the motor is operated. These DC-DC converters typically employ expensive high voltage switches that are rated for the DC link voltage.
It would therefore be desirable to provide an apparatus for coupling the low voltage ES to the high voltage DC link using less-expensive, low voltage switches.